Data which is sent and received in a communication network includes data which is sent by a data sending terminal, and an ACK (acknowledgement) which is used for informing the data sending terminal that the data has arrived normally.
A patent literature 1 describes that, in a communication system including a data sending terminal which sends data, and a data receiving terminal which receives the data, the data receiving terminal sends ACK, which indicates that the data has been received, to the data sending terminal. By receiving ACK, the data sending terminal can confirm that the data has arrived at the data receiving terminal. On the other hand, in the case that the data sending terminal does not receive ACK within a predetermined time, the data sending terminal judges that the data has not arrived at the receiving terminal, and then resends the data.
Moreover, a patent literature 2 discloses that, in the case that a received ACK is caused an error, a data sending terminal sends ACK resending request data which requests to resend ACK. A data receiving terminal which receives the ACK resending request data resends ACK. By adopting the configuration mentioned above, the data sending terminal receives the resent ACK even if the received ACK is caused the error, and consequently the data sending terminal confirms that the data has arrived at the data receiving terminal.